By analyzing the final moments of a comet that crashed into the solar …

Apparently, comets go crashing into the sun more frequently than I previously believed, with one grazing our stellar companion about every three days. The Large Angle and Spectrometric Coronagraph (LASCO), a piece of equipment aboard the Solar and Heliospheric Observatory (SOHO), has been in operation for 15 years, and it has watched more than 2000 comets approach the Sun. Only the largest of these comets, those with diameters of up to 100 m, have traversed their perihelion (the closest approach to the Sun) and survived to see another orbit. Comet C/2011 N3 (SOHO) was not one of the survivors. Fortunately, the Atmospheric Imaging Assembly aboard the Solar Dyanmics Observatory captured the last moments of this comet as it slammed into the Sun's inner corona, and the results have appeared in Science.

Comet C/2011 N3 (SOHO) was first seen on July 5, 2011 at 23:46 UTC when it was about 0.2 solar radii away from the Sun. Astronomers watched the comet's descent in five different wavebands in the extreme ultraviolet spectrum (EUV) for approximately 20 minutes before it faded into the Sun's atmosphere. During this final dive, it was traveling at approximately 600 km/s, which led to some blurring in the images and movies. In association with the paper, Science is hosting a pair of videos of the comet's last moments.

Using the EUV observations, the authors of the Science paper calculated the orbit of the comet and determined that it would have had a orbital perihelion of 97,200 km above the solar surface. This value is in good agreement with orbital parameters calculated using SOHO/LASCO and STEREO/SECCHI (Sun Earth Connection Coronal and Heliospheric Investigation) coronagraphs. This agreement suggests that the nucleus of the comet didn't slow down as it flew into the solar corona.

There was a feature visible in the absorption spectrum, just ahead of the comet's tail. That turned out to be the comet's coma—the dense region that surrounds the nucleus. It was seen about 7 arcseconds ahead of the tail when it crossed the edge of the sun (called the limb); at the distance of the sun, this meant that the coma was about 5000 km ahead of the bright part of the tail.

Using various absorption spectra features, the authors were able to determine that the radius of this dense leading region is 1400±600 km. However, due to the motion blurring and shutter speed of the camera, the authors consider this an upper bound. By making some assumptions about the opacity of the coma, they were able to narrow down the actual radius to somewhere between 50 and 700 km.

Using the time-lapse photos, it was possible to see a series of arches behind the comet's nucleus. These were interpreted as tracks of material that the comet shed as it plowed through the Sun's atmosphere. It was possible to estimate the rate of mass loss that produced these arches—the comet's orbital trajectory and speed were known, along with the time it took to decelerate in the corona and the corona's plasma density. For the 20 minutes the comet was seen falling into the star, it was losing between 106 and 108 g/s, corresponding to a total mass loss of between 6x108 and 6x1010g.

By doing some basic heat transfer analyses, the authors were able to determine that a single solid block of comet couldn't lose this amount of mass in this time window. Instead, the core must have been made up of numerous fragments. (A solid block would not have had enough surface area to possibly shed this amount of mass.) To make the heat transfer numbers work, the authors estimate that there were around one to two dozen fragments that were larger than 10 m in diameter.

This entire exercise proved to be a valuable new glimpse into the interior of comets. Previous estimates involved derivations based on changes in a comet's light curves as it orbited, and involved assumptions about its albedo and mass density. Direct observations made during a spacecraft flyby gave more information, but have been very rare. Given that our current observatories see a "Sun-grazing" comet on average once every three days, that should equate to several comets per year that survive long enough to plow into the solar corona. So, we'll hopefully be seeing more data of this sort in the future.

Matt Ford
Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems. Emailzeotherm@gmail.com//Twitter@zeotherm

Generally it has to be on an orbit that intersects somewhere with a deep gravity well. Say Jupiter. This then disturbs the orbit of the object (comet/rock/alien wreckage from the Oort cloud) and changes its course. Instead of whipping around the Sun at some unfathomable speed in something roughly approximating an oval, it starts heading towards the inner solar system.

If the slingshot doesn't cause the object to shed enough speed that it can find a comfortable orbit somewhere in the inner system, it will get caught into the Sun's gravity well directly. Here, one of two things can happen; either the object has a near miss (flinging it out into interstellar space) or it plunges into the fiery hell for the personal amusement of those apes on the third planet. (Oh, also, it delivers yummy hydrogen to Sol for it to fuse so that we have that femtosecond more life between now and when the sun hits the supergiant phase.)

The circumstances for this all to happen are pretty rare. When you think about all the “big” objects in our system - there are 8 planets and umpteen dwarfs – there is a lot of gravitational interaction that can happen. If Sedna knocks some godforsaken rock out of the Oort cloud and sends it barrelling towards the outer system, maybe it will get trapped by Neptune and settle into orbit, or even plunge to an icy doom.

Maybe that little rock will come trucking on towards the inner system at holyshit Kph and just sort of wander into Jupiter’s gravity well. This then might just slingshot it out into the general vicinity of “not here” and we don't get to watch it die an interesting fusion-plasma induced death.

Alternately, it could play some sort of weird gravitational ping-pong with all the various large bodies and slalom around the system until it settles on some really oblong orbit around the Sun that seems to defy physics but doesn’t.

Suffice it to say that the only reason we see so many of these things plunging into the sun is that there are rather a lot of the little blighters out there. Jupiter is generally pretty good at “shielding” earth from the bulk of them, which is really the only reason you exist at all. If we didn’t have our friendly neighbourhood fatman watching out for us, Earth would be a pockmarked sheet of glass with no atmosphere to speak of. Blasted off by all the crap barrelling out of the edge of the system.

Why does it all come flying out of the edge of the system? Because there’s lots of it there, and all these dwarf planets (Eris, Sedna, Makamake, etc.) just running around in the middle of all these debris fields churning it up and making life interesting.

Thank you for the awesome explanation. Must read several times and still digesting the full meaning. Your writing style is also awesome. You should (must ?) become science writer on Ars. Hope you would be OK to provide some extra explanations.

Astlor wrote:

Suffice it to say that the only reason we see so many of these things plunging into the sun is that there are rather a lot of the little blighters out there. Jupiter is generally pretty good at “shielding” earth from the bulk of them, which is really the only reason you exist at all. If we didn’t have our friendly neighbourhood fatman watching out for us, Earth would be a pockmarked sheet of glass with no atmosphere to speak of. Blasted off by all the crap barrelling out of the edge of the system.

Q1. Jupiter orbit is rather large compare to that of Earth. I suppose Jupiter cannot be always near the path of something far away rushing toward Earth. Is it correct to say that the Jupiter "shield" still have quite a lot of misses ?

Q2. "Blasted off by all the crap barrelling out of the edge of the system" : are these debris pushed to the edge? Or may be they were always there, not being able to form another bigger body?

Thank you for the awesome explanation. Must read several times and still digesting the full meaning. Your writing style is also awesome. You should (must ?) become science writer on Ars. Hope you would be OK to provide some extra explanations.

I would love to; it is one of my personal goals. However, there are some barriers to overcome. Firstly, I'm really not all that good yet; I haven't been writing professionally for even two years yet!

Secondly, Ars has a huge aversion to hiring people that don't live in the US. There are all sorts of bits of complicated legal and accounting poo that make such a thing difficult. For them to even consider me as a potential writer, I would have to come a long way past my current level of expertise.

I do write for The Register as a sysadmin blogger. It’s not science reporting. (It is technology/IT stuff.) This gives me a bit of a leg up; writing about "my day job" is a bit easier than trying to write about "my hobby" (science in general.) If you do like my writing, I can point you at some of my better articles. It might help you avoid the fluff and my (truly terrible) earlier works. (Please bear in mind that I am a sysadmin who is trying to learn to be a journalist…so some of my early stuff is pretty iffy, but I'm getting better. New careers take time!)

So…it’s not exactly science writing, but hopefully informative. (If you are an IT nerd.)

I think I am several years away from writing for any sort of science yet. I’ve learning to do.

BoCaro wrote:

Q1. Jupiter orbit is rather large compare to that of Earth. I suppose Jupiter cannot be always near the path of something far away rushing toward Earth. Is it correct to say that the Jupiter "shield" still have quite a lot of misses ?

Q2. "Blasted off by all the crap barrelling out of the edge of the system" : are these debris pushed to the edge? Or may be they were always there, not being able to form another bigger body?

They were always there. Remember that star systems form from dust clouds. The dust accretes into rocks which smash into other rocks…and you basically Katamari Damachi around until you reach notable size. Generally by the time you’re an asteroid, there isn’t much else around you. (No, asteroid belts as seen on TV bear no resemblance to reality. They are in fact quite void of matter; there is a great deal of distance from rock to rock.)

This is because by the time you are an asteroid, you have enough gravity to start pulling in dust, small rocks, etc. Eventually, you reach “dwarf planet” size, which means that you are round under the force of your own gravity, but there is still a lot of stuff floating around in your orbit that you haven’t yet nommed.

Getting larger, we get terrestrial (rocky) planets that have mostly cleared their orbits and tend to be pretty heavy for the volume of space they occupy. (That’s because they are rocks.)

There are also some little dinky icy planets that can get quite large (in diameter,) but don’t exert much gravity. (Because they are all gases. Methane, CO2, water, etc. Just frozen out and generally not very dense.)

Generally, a few giants will form. Ice giants like Neptune and Uranus, and gas giants like Jupiter and Saturn. These are the big mammas that ate the bulk of the gas in the system before anyone else had a chance to get to the buffet table.

Out at the edges of the system is generally where the gravity well of the star and all her larger bodies stops “meaning anything.” I.E. they don’t really pull on the dust/rocks/etc. Here you have the Kuiper Belt. This is basically a giant asteroid belt that surrounds the system.

Here you’ll find a great many smaller objects. Dust, rocks, smaller planetessimals and a great big wacktonne of dwarf planets. The reason is simple; they are far away from the big gravity wells, and the orbit they occupy is *huge*. (Think 60-100AU in diameter.)

So we are basically watching the solar system still forming out there. Because the objects are just so widely spread apart that they don’t interact all that often. What goes on in the Kuiper belt is a model of what the early solar system would have looked like, just with hugely magnified distances (and thus time scales.)

Get out futher than that and there is the Oort cloud. Here we’re almost a light year from the sun, and there is another asteroid belt. Except this time, there aren’t really any rocks. This is all frozen gases. The lighter stuff that “spun to the outside” when the system was forming.

Out here you have that gigantic douche known as Sedna. Sedna likes to make everyone’s life miserable. Sedna has a really bizarre orbit so it basically just whips through clouds of debris flinging them to and fro with nary a care in the galaxy. It will whip between the Kuiper belt and the Oort cloud and back again, on some elongated elliptical orbit, like a crazy dwarf planet comet of doom.

Sedna isn’t alone. We’ve found several dwarf planet candidates throughout the system, and there are many more to be discovered. The Kuiper belt alone is enormous; trying to spot all the large-ish bodies between here and the Oort cloud is looking for a cosmic needle in a haystack.

Every now and again, we might get a little bit close to another star system. That can send some stuff flying inward too, as other star systems can interact with the Oort cloud and cause things to shake up.

There are also rogue planets to worry about, and they are a lot more common than we had previously thought. Imagine a Jupiter-sized rogue (which we can’t see, because it doesn’t emit any light) just ploughing through the Oort cloud on its way from A to B. Knocking all this frozen crap inward, which hits various rocks and debris which hits other debris which interacts with various things gravitationally…

The universe is a messy place. It’s a good thing we have these large gravitational bodies to help clean it all up for us.

Hope that helps!

Edit: apparently all my links ended up being dupes. My sincere apologies to all. This has been fixed. I blame RDPclip for this error.

This comet, and the majority of the other Sun-grazing comets obsevered by SOHO are believed to be members of the Kreutz group (Wikipedia). This is a collection of material that have perihelions within 1-2 solar radii of the solar surface and orbital periods of between 500-1000 years. It is one of the largest known comet groups, and it believed to have been formed when its progenitor broke up somewhere around 2500 years ago. (Here's the paper from the Astronomical Journal on them (PDF))

This comet, and the majority of the other Sun-grazing comets obsevered by SOHO are believed to be members of the Kreutz group (Wikipedia). This is a collection of material that have perihelions within 1-2 solar radii of the solar surface and orbital periods of between 500-1000 years. It is one of the largest known comet groups, and it believed to have been formed when its progenitor broke up somewhere around 2500 years ago. (Here's the paper from the Astronomical Journal on them (PDF))

I should also add that the aphelion (the farthest-away point of the orbit from the sun) is somewhere between 160 and 190AU for this cometary group. Meaning that these things have a long elliptical orbit that starts out 160-190 times the distance between the earth and the Sun (that means way out there; between the Kuiper belt and the Oort Cloud,) and then they whip around the sun coming in some cases as close as 0.0013AU (200,000km) to the star itself.

Basically, they slowly head in towards the inner solar system. They pick up speed as the gravity of Sol tugs on them continuously. By the time they are somewhere around Jupiter they are absolutely screaming through the system. They whip around Sol with blistering speed, and the star's gravity slingshots them back out towards the fringes of the solar system. Here they slow right down to almost nothing before completing the orbit and heading back in towards the system again.

Understand the speeds we are talking about here. 230km /sec. That’s over 500,000 miles per hour. At that speed, it would take one of these comets only 7.5 days to cover the distance between earth and the sun. Now; imagine if one of these little blighters – say 0.25km across or so – slammed into the next town over.

Oh wow, thanks for the patience to write the detailed answer. Sorry for wasting your time, I forgot to mention that I have a decent understanding of the mechanism of the formation of the solar system. I misunderstood your original answer and thought that most of the debris in the Kuiper Belt were formed by comet debris and pushed out to the edge (around the Kuiper Belt orbit) and stabilized somehow on that orbit.